He hardens whom He wants to harden: Pharaoh refused to let Israel leave because his heart became hard. A hard heart describes stubborn resistance to God. It means refusing to listen or change, even after seeing clear signs of God’s power.
The book of Exodus says that God hardened Pharaoh’s heart (Exodus 9:12). It also says that Pharaoh hardened his own heart (Exodus 8:15). Together, these statements show that Pharaoh was responsible for his actions, even as God carried out his purposes through Pharaoh’s refusal.